Yeast are micro-organisms that are more closely related to a mushrooms (they are a single-celled fungi) than to bacteria. Fungi are not green, do not have chlorophyll, and cannot undergo photosynthesis to make their own food. Instead, yeast must get their food from their surrounding environment. Yeasts use sugars (simple carbohydrates) and starches (complex carbohydrates) as food sources.Yeast break down these sugar molecules to make energy and release carbon dioxide gas (CO2) as a result. The carbon dioxide gas is what makes a slice of bread so soft and spongy. The pockets of gas are produced by yeast when the dough is allowed to rise before baking. This is the process of fermentation. Fermentation is also used to make beer, wine, and champagne. In this lab we will be examining how the presence of sugar changes how yeast is able to ferment and produce carbon dioxide gas.